1. Field of Invention
This invention relates to a method of and apparatus for use in carrying out structural surveys. The invention is of particular advantage in photographic tunnel sectioning, but it is applicable also to the surveying of other structures such as pipelines, e.g. sewer pipes, bridges and ventilation shafts. In the interests of brevity and clarity the invention will be described herein in relation to the photographic sectioning of railway tunnels. It should be understood, however, that this application of this invention is given by way of non-limiting example only.
Description of the Prior Art
In the past, tunnel sectioning has been carried out using stereo-photogrammetric techniques. Although the results are satisfactory this method has the disadvantages of being expensive and time consuming.
It has been proposed to optically produce photographic records of tunnel profiles by projecting a plane onto the tunnel walls and photographing the illuminated line contour. The image obtained is a graphical representation of the tunnel section at that plane. Although this method has been tested it has met with limited success and it has not been taken into general use. The light plane was generated by sandwiching a photographic ring flash between a pair of reflective plates. The plane of light emitted by this device is very coarse so that the line quality and contrast are poor and the lack of resolution in the photographic image makes accurate assessment of deformations in the tunnel contour impossible.
The present invention seeks to overcome the drawbacks of the methods described above and according to the invention there is provided a structural surveying method wherein a plane of light is emitted by a device and impinges on the structure being surveyed to produce an illuminated line profile of the structure in the light plane. The light emitting device includes a housing defining a chamber having a peripheral wall with a first circumferentially extending light transmitting aperture, a second circumferentially extending light transmitting aperture spaced outwardly from and coplanar with the first aperture, and a light source for emitting light into the chamber. A substantially planar beam of light is thereby transmitted outwardly through the apertures when the light source is illuminated.
The illuminated line profile is preferably recorded photographically and the photographic image may be analyzed numerically. The numerical analysis may include the steps of scanning the profile image, digitizing the coordinates of selected points on the line image, applying scale corrections to the digitized coordinates, and computing parameters representative of the structural geometry.
In accordance with another aspect, the invention provides an apparatus for use in carrying out structural surveys, including a device for emitting a plane of light to produce an illuminated line profile of a structure being surveyed. The device includes a housing enclosing a chamber having a peripheral wall, a first circumferentially extending light transmitting aperture in the peripheral wall, a second circumferentially extending light transmitting aperture spaced outwardly from and coplanar with the first aperture, and a light source arranged to illuminate the interior of the chamber for light emitted therefrom to be transmitted through the apertures and project therefrom as a substantially planar beam.
The double aperture arrangement has been found to produce a plane of light of adequate intensity for surveying railway tunnels and at the same time with sufficient resolution to permit photographic recordal of the tunnel sections and on-line numerical analysis of the photographic images. If required three or more coplanar apertures could be used.
In a preferred device according to the invention the housing has a non-reflective interior surface; the apertures extend substantially entirely around the chamber; the light source is a photographic ring flash and/or halogen lamp mounted within the chamber; and, each aperture is defined between a pair of opposed lips parallel to the plane of the apertures.
Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description when considered in connection with the accompanying drawings, in which like reference characters designate like or corresponding parts through the several views and wherein.
FIG. 1 is a schematic cross-section through a preferred device for emitting a plane of light according to the invention;
FIG. 2 shows the device of FIG. 1 mounted on a railway trolley to facilitate movement into and along a railway tunnel;
FIG. 2a is a side view showing a detail of the trolley mounting;
FIG. 3 is a perspective view of the complete apparatus;
FIG. 4 depicts a photographic negative image of a tunnel profile obtained using the method of the invention; and
FIGS. 5a, 5b, 6a and 6b are graphs obtained by plotting Radii and Versine differences for tunnels with and without distortions.